How to Make It
In a large pot, heat the oil. Add the onion and cook over moderately low heat until softened, 5 minutes. Add the garlic and cook, stirring, until fragrant, 2 minutes. Add the tomatoes and bring to a simmer. Stir in the wine and cook over moderate heat for 2 minutes. Season with salt and pepper; set aside.
In a saucepan, bring the water, stock and milk to a boil. Gradually whisk in the polenta. Cook over moderate heat, whisking, until the polenta is slightly thickened, 2 minutes. Stir in 2 tablespoons of the butter and season with salt and pepper. Cover and keep warm.
Bring the tomato sauce back to a simmer. Add the shrimp and squid and simmer over moderately high heat until barely cooked, about 2 minutes. Add the mussels, cover and cook until they open, 4 minutes. Swirl in the remaining 1 tablespoon of butter and the parsley. Season with salt and pepper.
Whisk the polenta and ladle it into bowls. Spoon the stew over the polenta and serve.